typedef struct _KGUARDED_MUTEX
{
     LONG Count;
     PKTHREAD Owner;
     ULONG Contention;
     KGATE Gate;
     union
     {
          struct
          {
               SHORT KernelApcDisable;
               SHORT SpecialApcDisable;
          };
          ULONG CombinedApcDisable;
     };
} KGUARDED_MUTEX, *PKGUARDED_MUTEX;

Windows Vista Kernel Structures